Many diverse observations suggest that extracellular, as well as intracellular, thiol-disulfide reactions are associated with cell growth control and that viral cell transformation results in changes in such phenomena. A major hindrance to further investigation of the importance of such observations is the lack of a quantitative, sensitive, and specific analytical method for analysis of low molecular weight thiol-disulfide components. The specific aims of the present studies are: (1) to develop a general method for analysis of thiols, and thereby disulfide components, based upon isolated of thiols by affinity to a Sepharose-linked mercurial, conversion of the thiols to mixed disulfides of cystine, and quantitative measurement of the latter by conventional amino acid analysis; (2) to utilize this methodology to analyze the thiol, disulfide, and protein mixed disulfide content of two lines of normal cells (3T3 and MDCK), of two corresponding lines of virally transformed cells (SV40-3T3 and MSV-MDCK), and, simultaneously, of the growth media during exponential growth at low cell density and during growth at high cell density (confluent cells).